Since herbal tea consists of 99% water, it is advisable not to be careless when choosing your water.
In general, we can say that tap water in most cases is not a good option for an optimal tea experience. Especially not in Belgium and the Netherlands because the mineral content (especially calcium and magnesium) of tap water there is very high.
Minerals ensure that the taste and active ingredients of the tea herbs cannot be properly absorbed. Furthermore, there are many other disadvantages for the tea: unpleasant aroma, dark turbid color, an unpleasant film on the water surface, limescale in your tea cup or teapot.
There are various ways/systems to purify your water and remove chemical substances and high mineral concentrations. The use of filter jugs (e.g. Brita Maxtra) generally provides good results in terms of price/quality according to us.
Buying low-mineral bottled water can also be a good solution, although this option is usually a bit more expensive. Examples of low-mineral water are Spa Reine, Mont Roucous, Montcalm and Montille.